1. Field of the Invention
The present invention relates to a technology for processing image data including a first image component corresponding to image-forming light and a second image component corresponding to non-image-forming light, and more particularly, to an image processing technology for making the second image component less conspicuous.
2. Description of Related Art
A diffraction optical element which uses the diffraction phenomenon of light generates chromatic aberration completely opposite to that of a conventional dioptric element (a refraction optical element). In other words, its dispersion has a negative value. Therefore, in addition to the dioptric element when an optical system is comprised by using a diffraction optical element, the optical system can be made more compact and lightweight than an optical system comprised by using only the dioptric element while chromatic aberration is corrected properly.
However, among the multiple orders of diffraction light generated by incident light to the diffraction optical element, only one order component is used for image-forming in the optical system. Diffraction light of another order components is unnecessary diffraction light and appears as flare on an image surface, thereby deteriorating the quality of the image.
To this problem, a technology has been proposed in which diffraction optical elements 201 and 202 having diffraction surfaces 201a and 202a, respectively, are laminated as shown in FIG. 10, so that the diffraction efficiency of unnecessary diffraction order light (hereinafter referred to as unnecessary diffraction light) is significantly reduced in comparison with a conventional single-layer diffraction optical element. By using this kind of laminated diffraction optical element (DOE) 200, it is possible to accomplish a camera lens which has high image-forming performance and being compact while chromatic aberration is corrected properly.
However, even when the DOE in which the diffraction efficiency of unnecessary diffraction light is reduced sufficiently is used, in the case when an object which has very intense luminance is taken, an amount of unnecessary diffraction light is raised by the intense amount of the light and increases. Thereby, a flare image according to the unnecessary diffraction light becomes a major factor for deteriorating image quality.
Several attempts have been done to reduce or correct this kind of deterioration in image quality by using a digital image processing. Japanese Patent Application Laid-open No. H9(1997)-238357 (the. U.S. equivalent is U.S. Pat. No. 6,088,059) and Japanese Patent Application Laid-open No. H11(1999)-122539 (the U.S. equivalent is U.S. Pat. No. 6,452,635) have proposed a technology of correcting the flare image according to the unnecessary diffraction light in a digital image-taking system which uses an optical system including a diffraction surface by calculating the flare component based on the unnecessary diffraction light generated on the diffraction surface using a point spread of this image-forming optical system and by subtracting the flare component from the taken-image.
Generally, the diffraction efficiency of the light which has the diffraction order used for image-forming on a DOE (hereinafter referred to as image-forming diffraction light) is sufficiently higher than that of the unnecessary diffraction light. Thereby, flare according to the unnecessary diffraction light rarely adversely affects image quality.
However, in the case when an object having such high luminance as to saturate the luminance value of the pixel signal of an image-pickup device for use in a digital camera is taken, the unnecessary diffraction light appears as flare on the image surface. In this case, the technology proposed by the Japanese Patent Application Laid-open No. H9(1997)-238357 and Japanese Patent Application Laid-open No. H11(1999)-122539 cannot accurately calculate the flare image component since the luminance value of the pixel signal of the image-pickup device is saturated.
In addition, the highlight which causes flare is not always limited to an ideal point-source light shown in FIG. 1(a), but has a two-dimensional distribution shown in FIG. 1(b). Furthermore, the distribution differs depending on the photographing environment. Hence, it is difficult to accurately estimate the flare image component.
Moreover, as proposed in the Japanese Patent Application Laid-open No. H9(1997)-238357 and the Japanese Patent Application Laid-open No. H11(1999)-122539, in the case when the obtained flare image component is simply subtracted from the taken-image, the luminance value at the edge of the subtracted region becomes discontinuous, and a visually unnatural image is obtained.